Crystal contacts of which one element is silicon



. hitherto.

- sary fragments, -'--taken.'-

l atented May 6, 1947 CRYSTAL CONTACTS OF WHICH ONE ELEMENT IS SILICON Charles Eric Ransley, Sudbury, John Walter Hyde, London W. 14, and Stanley Vaughan Williams, Kenton, England, assignors to The General Electric Company Limited, London,

England No Drawing. Application June 8, 1942, Serial No. 446,310. In Great Britain May 28, 1941 '2 Claims.

This invention relates to electrical crystal contacts of the type in which an element is silicon; the other element is usually a metal point. Such contacts can be used as rectifiers of alternating current or as mixers, that is to say non-linear impedances to which are applied an oscillation of frequency f1 and another oscillation of frequency f2, so that oscillations are produced having frequencies pfizL-qfz, Where p and q are integers.

Objects of the invention are to provide improved processes for the preparation of silicon elements of such crystal contact devices, and crystal contact devices embodying such silicon elements, that are more uniform or more efficient or both than those of the said type known The uniformity aimed at means that th electrical characteristic of the contact should be as nearly as possible independent of the position on the silicon of a metal point forming the other element of the contact. Efiiciency generally requires a high ratio of impedance in one direction (reverse impedance) to impedance in the other direction (forward impedance). Hereinafter when impedance is mentioned without the qualification forward or reverse, the forward impedance is meant. It may also require that the absolute values of the forward impedance should be high or, alternately, that they should be low. It may also require that the forward direction should be that in which positive charge flows from the silicon to the metal (positive contact) or that it should be that in which negative charge flows from the silicon to the metal (negative contact).

a contact promotes the obtaining of uniformity and efiiciency of performance of the contact.

' Effective purification of commercial silicon involves the step of grinding the silicon to a fine powder, and in this form the pure silicon is incapable of use in contacts, in which relatively large fragments are required. We have discovered that it is possible to use such a purification process without contaminating the silicon in the subsequent step of melting it to yield the necesprovided suitable precautions are According to the invention, the manufacture of the silicon element of a crystal contact of the type specified comprises the steps (1) grinding relatively impure silicon to a fine powder and treating it with chemical reagents capable of removing substantially all the impurities usually present in commercial silicon, (2) melting this pure product, out of contact with the atmosphere, and while contained by pure beryllium oxide.

No novelty is claimed for either of these steps per se; a novel feature of the invention lies in combining the two steps to form one stage in the manufacture of the silicon element of a crystal contact of the type specified. 7

In carrying out step (1) aforesaid We have found suitable the process described by N. P. Tucker in the Journal of the Iron and Steel Inst., vol. CXV (1), p. 412, 1927. I

It is probably impossible to find any container for melted silicon that will not react at all with the silicon. But we have found containers of two kinds that react so little that, if the product is not retained in the container longer than is necessary to secure complete melting, they produce no contamination, that is to say, they introduce no impurity that is prejudicial to efiiciency of the final contact. One of these containers is a crucible made of or lined with pure beryllium oxide. The other is a tantalum crucible coated with tantalum carbide by heating it to about 2000 C. in naphthalene vapour. It is important that the coating of carbide should be continuous, e. g., free from pin holes. We know of no reason why any other container should be used.

In order to melt the silicon the crucible may be heated by high frequency currents induced either in it (if it is of metal) or in a metal (e. g., molybdenum) cylinder surrounding it. As already indicated, the heating should be stopped as soon as melting is complete.

The character of the resulting contact depends to some extent on the gas in contact with the silicon during the melting. If a low impedance positive contact is required, the density of the surrounding gas is preferably negligible; e. g., the gas should be residual gas at a pressure of less than 10- mm. We have found that presence of a little oxygen in the surrounding gas is not always deleterious, especially if a negative, rather than a positive, contact is desired. However, the effects of oxygen are extremely complicated and have not been completely analysed.

The uniformity of the contact is often promoted by polishing the siliconwhere it is to contact with the metal, for example, by grinding it on emery cloth of increasing fineness and finally bufling it to a high polish. But occasionally it may be desirable to. usethe rough broken surface of the melt; for then a'point is more likely to keep its position.

Substantial and sometimes desirable modifications of the properties of the silicon may be made by treating the surface, where it is to -contact with the metal, with hydrofluoric acid, e. g., by immersing it for 10 seconds .in a mixture of equal parts of pure hydrogen fluoride (40% and water.

In the experiments whose results are -iven hi? low the point associated with the silicon wasthe end of a tungsten wire, 0.2 mm. in diameter, sheared along the cleavage plane which is in-. clined at approximately 45 to the axis of the wire. The pressure between the .point and the silicon was 10 to 15 gm. weight. The E. 'M. F. applied was 1.5 volts direct. The silicon was melted in a carbideecoated tantalum crucible.

If the point isin contact with anatural cleavage plane of the crystal, the efiect of the acid is usually to decrease the impedance and to increase the aforesaid ratio, unless possibly that ratio is already high. Two examples of the efiect are given in Table I.

Table I.Natural cleavage surface ,Beiore HF After HF treatment treatment Forward Impedance ohms 415 273 198 208 Reverse Impedage Forward Impedance 2- 5 15- 5, 12. 7 18. 9

If the point is in contact with a polished surface, the impedance is less afiected; but the efiect on the ratiois similar. Two examples are given in Table ll.

Table Il .PoZished surface BeforeH'F After HF t ea me r atmen Forwardlmpedance 233 264 245 238 Reverse Impedance J ff jT m 12.8 18.4 17.0 17.7

Table .111

Before imer Ne'OH NaOII treattreatm nt -men Forward Impedance, .210. 933 Paw Forward Impedance l5- 5 15.0

The effect of caustic soda on-a1polished-surface appears to be -less.

We claim:

1. "In'the manufacture of an electrical'crystal contact device 01' the kind 7 in which :a contact element thereof is silicon, the production of the said silicon contact element by a process which comprises the steps (1) grinding relatively impure silicon-to a. fine powder and tre t ng sa d p w with chemical reagents capable of removing substantially all the impurities usually present in commercial silicon, (2) melting the pure product of such treatment, out of contact with the atmosphere, and while contained by pure beryllium oxide.

'2. In the manufacture of an electrical crystal contact device of the kind in which a contact element, thereof-is silicon, the production of the said silicon contact element by a process which comprises the steps (1) grinding relatively impure siliconito afine powder and treating said powder with chemical reagents capable of removing substantially all the impurities usually present in commercial silicon, (2) melting the pure product of such treatment in the presence of residual gas atta pressure of less than 10* mm., and while contained by'material with which said pure product does notlreact chemically in such a manner ,as to become contaminated undesirably,;and 18) after solidification of said molten product oxidizing at least that'face of said contactelement which is to be engaged by the point contact ele- -ment of said device.

3. In the manufacture of an electrical crystal contact device of the kind having two co-operating contact elements, one of said contact elements being of silicon, the production of said silicon contact element by a process which comprises (1) grinding relatively impure silicon to a fine powder and treating said powder with chemical reagents capable of removing substantially all the impurities usually present in commercial silicon, 2) melting the pure product of such treatment, out of contact with the atmosphere,-and in a container or such a composition that said pure product does not react chemically with it in-sucha manner as to become contaminated undesirably, and after solidification-said molten product'treating a natural cleavage surface thereoi with hydrofluoric acid, and thereafter disposing the other of said contact elements in contact with said etched 18 cc.

4, In. the manufactur of a ele ca r stal contact device of the lgind having two co-cperatins contact elements, one of sa d contac 1 inents being of si on, heproduction of said silicon contact element by a process which comprises '(1)- grinding relativel impure silicon to a fine powder and treating said powder with .chemicalreagents ,capable of removing substantially all the impurities usual y p sen in comm rcialsilicon, iziflmelti 'e ure p od -0 su h reatment, ou o c nta t wi he atmosphere, andina container of such a composition that a P e p duct does not react chemically with it in such a manner as tobecomecontaminated undesirably, and after solidification of said molten producttreating a face thereof with an oxidizin a en and the dispos n the other of said contact elements in contact with said treated face.

5, Inthe manufacture ofan electrical crystal contact device of the-kind having two co-operatne contac l m n a on o a d, n c ments ei g-0i li n, the p duct n of sa si icon ont ct e emen by apmcess which co prises (l) :grin ns rela ive y mpure si icon .to a dine :powd r and treatin :said owder with chemical reagents capable of remo tially all the impurities usually present in commercial silicon, (2) melting the pure product of such treatment, out of contact with the atmosphere, and in a container of such a composition that said pure product does not react chemically with it in such a manner as to become contaminated undesirably, and after solidification of said molten product treating a natural cleavage surface thereof with a caustic alkali solution, and thereafter disposing the other of said contact elements in contact with said treated surface.

6. In the manufacture of an electrical crystal contact device of the kind having a silicon contact element co-operating with a metal contact element, the preparation of said silicon element by steps comprising grinding relatively impure silicon to a fine powder and treating said powder with chemical reagents capable of removing substantially all the impurities usually present in commercial silicon and melting the pure product of such treatment in a gaseous medium at a pressure of less than 10- mm. and in a container of such a composition that said pure product does not react chemically with it in such a manner as to become contaminated undesirably, and after solidification of said molten product, treating a natural cleavage surface thereof with an etching reagent, and thereafter disposing said metal contact in contact with said treated surface.

7. In the manufacture of an electrical crystal contact device of the kind having a silicon contact element cooperating with a metal contact element, the preparation of said silicon element by steps comprising grinding relatively impure silicon to a fine powder and treating said powder with chemical reagents capable of removing substantially all the impurities usually present in commercial silicon and melting the pure product of such treatment in a gaseous medium at a pressure of less than 10- mm. and while contained by pure beryllium oxide, after solidification of said molten product, polishing a face thereof, and treating said polished face with an etching reagent, and thereafter disposing said metal contact in contact with said polished and etched face.

CHARLES ERIC RANSLEY.

JOHN WALTER RYDE.

STANLEY VAUGHAN WILLIAMS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

